Catalysts comprising platinum group metals (e.g., platinum and palladium) supported thereon are known to have high capability to remove hydrocarbons from exhaust gas by oxidation. For example, Japanese Unexamined Patent Publication No. 106691/1976 describes a catalyst for purifying exhaust gas, the catalyst comprising palladium and platinum supported on an alumina carrier. However, even these catalysts fail to produce sufficient purifying effects on exhaust gas containing hydrocarbons which mainly consist of methane having high chemical stability, like exhaust gas resulting from natural gas combustion. Unlike ammonia, methane that is substantially non-toxic and has poor photochemical reactivity is considered not to greatly damage the regional air environment. However, it is desirable that methane emissions be suppressed as much as possible from the viewpoint of future global environmental protection.
It is also known that reaction inhibitors such as sulfur oxides usually coexist in exhaust gas and these compounds greatly reduce catalyst activity with time. Unlike petroleum fuel such as kerosene or light oil, natural gas does not originally contain substantial amounts of sulfur compounds. However, city gas derived from natural gas and supplied in our country further comprises a compound containing sulfur as an odorant, which forms a sulfur oxide during the combustion of gas, inhibiting catalytic activity of platinum group catalysts.
For example, Lampert reports that when methane is oxidized with a palladium catalyst in the presence of only 0.1 ppm of sulfur dioxide in the methane, the catalyst activity is almost totally lost in several hours (Applied Catalysis B: Environmental, vol.14, pp.211-223 (1997)).
Yamamoto et al. report that when hydrocarbons are removed from combusted city gas using a catalyst comprising palladium and platinum both supported on alumina, catalyst activity sharply declines in a short time of about 100 hours (Catalysis Society of Japan Meeting Abstract, 1996, published on Sep. 13, 1996).
Further, Japanese Unexamined Patent Publication No. 332392/1996 describes an oxidation catalyst for removing low-concentration hydrocarbons from exhaust gas in the presence of an excess of oxygen, the catalyst comprising at least 7 g/l of palladium and 3 to 20 g/l of platinum both supported via an alumina carrier on a honeycomb substrate. However, since this catalyst does not have sufficient long-term durability, time-dependent degradation of catalytic activity is unavoidable.
As described above, the conventional exhaust gas treatment methods have the problems that methane removal efficiency is low and that catalyst activity sharply declines in a short time in the presence of sulfur oxides.
Exhaust gas usually contains nitrogen oxides in addition to hydrocarbons. Known methods for removing nitrogen oxides and hydrocarbons from exhaust gas include, for example, a method for removing nitrogen oxides using hydrocarbon as a reducing agent (e.g., Japanese Unexamined Patent Publication No. 310742/1989).
Further, Japanese Unexamined Patent Publication No. 90826/1992 describes a method for purifying exhaust gas, comprising bringing exhaust gas containing nitrogen oxides into contact with a specific catalyst in the presence of hydrocarbon in an oxidation atmosphere containing an excess of oxygen so as to decompose nitrogen oxides and bringing the exhaust gas into contact with an oxidation catalyst so as to suppress hydrocarbon emissions.
According to the methods described in the above publications, propane, propene or the like converts to carbon dioxide at a high rate under conditions suitable for the removal of nitrogen oxides, so that only a low percentage thereof remains in the treated gas. However, a large amount of methane unavoidably resides in the treated gas because of its low reactivity under the conditions where known catalysts for removal of nitrogen oxides exhibit high activity. Therefore, in the case of treating exhaust gas containing methane, improvement is necessary in this point.
Japanese Unexamined Patent Publication No. 90826/1992 describes removing unreacted hydrocarbons by passing exhaust gas through a nitrogen oxide-removing catalyst and thereafter through an oxidation catalyst, and mentions, as oxidation catalysts, catalysts comprising 0.01 to 2% of platinum, rhodium, palladium or the like supported on porous carriers. The publication, however, nowhere describes reactivity of these catalysts to methane. In view of the above-mentioned report of Lampert et al., it is hardly conceivable that such a catalyst is capable of removing methane from exhaust gas.
U.S. Pat. No. 5,260,043 describes a method for removing nitrogen oxides and methane from a gas containing methane, nitrogen oxides and oxygen, the method comprising using a Co-ZSM-5 catalyst in the first stage and using a Pd-ZSM-5 catalyst in the second stage. However, practicability of this method is extremely questionable because neither steam nor sulfur oxides are contained in the gas tested according to the method described in the publication. Furthermore, in consideration of descriptions about effects of steam on methane-oxidation activity of Pd-ZSM-5 catalysts provided in another publication (Yuejin Li and John N. Armor, Applied Catalysis B: Environmental, vol.3, pp.275 (1994)), it is unlikely that methane will be removed from actual exhaust gas containing steam, by the method disclosed in the patent.
The above is summarized as follows. It is difficult to remove methane from actual exhaust gas by oxidation using any prior art technique, by treating the gas within a temperature range where nitrogen oxide-removing catalysts act effectively. It is difficult to achieve a high hydrocarbon conversion rate in the case of treating exhaust gas containing hydrocarbons containing a large amount of methane, for example, exhaust gas resulting from natural gas combustion. Consequently, if natural gas is added to exhaust gas to reduce nitrogen oxides, methane emissions will increase.
Other sources of methane-containing exhaust gas include organism-derived, methane-containing exhaust gas generated by fermentation of garbage, domestic animal wastes and like biological wastes. Such exhaust gas, however, usually contains sulfur-containing organic compounds derived from protein, etc. Therefore, during the removal of hydrocarbons by oxidation with a catalyst, the sulfur-containing organic compounds are oxidized to sulfur oxides, which reduce catalyst activity in a similar manner as described above.